1. Field of the Invention
The present invention relates to a radio communication system, and more particularly to a radio communication system which includes a plurality of base stations which communicate with a plurality of radio terminals respectively over radio waves.
Examples of such a radio communication system include a digital cordless telephone system, a wide area digital cordless telephone system which is called a personal handy phone system (PHS) in Japan and a personal communications services system (PCS) in the United States, a cellular phone system, a wireless printer system, and a wireless local area network system.
2. Discussion of the Background
Radio communications systems are increasingly used for wireless mobile communications.
An example of a radio communication system is a cordless telephone system which is used by individuals in home or office. A cordless telephone system includes at least one telephone base set which communicates with a radio mobile telephone set over a limited distance, such as several tens of meters, for example, by use of radio waves. The base station is electrically connected to a conventional public or private wire telephone network. The base station allows the user of the mobile telephone set to access the wire network while moving from one place to another or from a place distant from the base station in home or office as long as the mobile telephone set is within an area approximately several tens of meters from the base station.
In a cordless telephone system, a specific range of frequencies is assigned to each system and a mobile telephone set in a system can only communicate with a base station in the same system utilizing the same specified range of frequencies.
As understood by those skilled in the art, because only a limited number of frequencies are available for radio communication, the same range of frequencies must be assigned to and used by a plurality of cordless telephone systems. If two systems having the same range of frequencies are placed and used in close proximity to each other, such as in an area within several tens of meters of each other, a radio wave from one base telephone set may interfere with a radio wave from the other base telephone set. As a result, various problems may occur when receiving and sending calls with each of the mobile telephone sets. For example, an incoming call arriving at base telephone set of one of the systems, besides reaching a mobile telephone set within its system, may also reach a mobile telephone set of another system.
Therefore, when one system has a same range of frequencies as another system, the two systems need to be placed in sufficiently separated areas for avoiding such an interference problem.
Recently, radio communication systems have been proposed which allow the user of a mobile telephone set of a cordless telephone system to access a wire network not only in home or office but also in public places, outside of the home and office, such as on streets in business and shopping quarters.
Such wide area radio communication systems include a plurality of base stations. Each of the plurality of base stations is capable of communicating with a plurality of mobile radio terminals over a limited distance.
Generally a certain spectrum of frequencies is allocated for use in a wide area radio communication system. Because of the limited number of radio frequencies which are available in the allocated spectrum, the same plurality of ranges of frequencies within the allocated spectrum are allocated to and used by each of a plurality of base stations, and each of the plurality of base stations is placed in an area separated from each other, thus covering a wide area.
An example of such a wide area radio communication system is a system called DECT (Digital European Cordless Communications) in Europe. Another example is a personal handy phone system called PHS in Japan. Similar systems have been proposed in the United States and are referred to as personal communications services (PCS) systems.
Recent wide-area radio communication systems, such as the personal handy phone system (PHS) and the personal communications services (PCS) systems, provide mobile radio voice, data, video and/or multimedia communications using mobile radio terminals, which include a radio telephone, such as a cellular telephone, and other components for voice, data, video and/or multimedia communications.
In the personal handy phone system (PHS), as an example of such a wide area radio communication system including a plurality of base stations which communicate with a plurality of mobile radio terminals, each of the base stations is electrically connected to conventional public and private wire telephone networks and is capable of simultaneously communicating with a plurality of mobile radio terminals over a limited distance, such as for example, several tens of meters. In the PHS system, the base station is called a cell station and the mobile radio terminal is called a personal station, and are hereinafter called as such when referring to the PHS system.
For accomplishing multiple radio access and transmission between a cell station and a plurality of personal stations, a Time-Division Multiple Access (TDMA) architecture and a Time Division Duplex (TDD) architecture are used in the PHS system.
A digital signal carried by a radio wave which is radiated by each of the cell stations and the personal stations is divided into 5 ms time segments, and each segment is defined as a TDMA frame.
Each frame is further divided into 8 time slots. Accordingly, the time allocated for each slot is 625 .mu.s. Of the 8 slots, four slots are assigned for downlink (transmission from a cell station to a personal station) channels and four slots are assigned for uplink (transmission from a personal station to a cell station) channels.
One slot of each four slots is used as a slot for setting function channels for transmitting control information necessary for controlling the connection between the cell station and the personal station and is called a control channel. Function channels for transmitting control information include, for example, a broadcast control channel (BCCH) for broadcasting control information, and a common control channel (CCCH) for transmitting control information necessary for call connection between the cell station and the personal station and so forth. The control channel is commonly used by each of the personal stations.
The other three slots each are used as slots for communication and are called communication channels. Each of the communication channels is allocated to and used by an individual user (a personal station).
The cell station broadcasts to all personal stations, with the broadcast control channel (BCCH), control information related to a channel structure and system information such as information regarding the slots which are available for transmitting control information for obtaining a communication channel. A personal station receives such information and transmits, for example, information for informing its current location to the system which is necessary for receiving a call addressed to the personal station.
In the PHS system, there are provided three communication protocol phases, a phase for establishing the radio interface handshake, which is called link channel establishment phase, a phase for connecting a call between the cell station and the personal station which established the handshake, which is called a service channel establishment phase and a phase for performing communication and data transmission, which is called a communication phase.
The link channel establishment phase is defined as the stage for using control channel functions to select a channel (link channel) with the quality and capacity required for each call connection.
Function channels used in the link establishment phase are called a logical control channel (LCCH). As downlink logical control (LCCH) elements, there are a broadcast control channel (BCCH), a paging channel (PCH) which is a one-way point-multipoint channel that simultaneously transmits identical information to individual cell stations and a signaling control channel (SCCH) which is a bi-directional point-point channel that transmits information needed for call connection between the cell station and a personal station.
Each of the cell stations intermittently transmits the control channel signal which indicates a structure of the logical control channel (LCCH) transmitted by the cell station and the positions of the control channel with which transmission is possible for each of the personal stations.
The minimum cycle of the downlink logical control channel (LCCH) specifying the slot position of all logical control channel (LCCH) elements is called a LCCH superframe.
In a radio communication system including a plurality of base stations which communicate with a plurality of radio terminals, such as the above-described PHS system, when a base station is located in the vicinity of another base station and both base stations broadcast a control channel signal at the same time, for example, a problem occurs because the broadcasting radio waves interfere with each other, since a same range of frequencies is used by each of the base stations. If such interference occurs, radio terminals can not receive the broadcasting radio wave and consequently the radio terminals cannot communicate with the base station. As a result, a call addressed to a radio terminal and arriving from a corresponding base station may not be received by the radio terminal or a call originating from the radio terminal may not reach the base station.
For avoiding occurrence of such interference of control channel signals among a plurality of base stations, a technology exists to randomly change a timing at which a control channel signal is broadcast and is disclosed in Japanese Patent publication Toku-kai-hei No.7-75166. However, even if the timing for broadcasting the control channel signal is changed at random, the occurrence of interference of the control channel signals is not completely avoided since the changed timing for broadcasting the control channel signal might still coincide with the timing for broadcasting the control channel signal by one of the other base stations.
As the number of base stations in the system increases, the amount of traffic on the control channel signal increases and consequently the frequency of occurrence of interference of the control channel signals increases. This may result in a situation in which a base station can not broadcast the control channel signal due to busy traffic on the control channel signal.
In an attempt to solve such a problem of interference of the control channel signals, a cordless telephone system which is used in an office, as an example of a radio communication system including a plurality of base stations which communicate with a plurality of radio terminals, can provide two control channel signals at different frequencies. Each base station is allocated one of the two control channel signals.
However, in such a system utilizing two control channel signals, as the number of base stations increases, a situation may occur in which the traffic on at least one of the two control channel signals increases and becomes busy while the traffic on the other control channel signal is relatively light. This is because each base station is allocated one of the two control channel signal frequencies and such allocated control channel signal can not be changed. Therefore, even with such a system using two control channel signals, interference of control channel signals among a plurality of base stations still occurs when the plurality of the base stations are located in close proximity to each other.